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No. 8878 





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Bureau of Mines Information Circular/1982 



Chemically Bonded Refractories— 
A Review of the State of the Art 



*>@i 




UNITED STATES DEPARTMENT OF THE INTERIOR 



^Ws.S^>i 



Information Circular 8878 



Chemically Bonded Refractories— 
A Review of the State of the Art 



By Rustu S. Kalyoncu 




UNITED STATES DEPARTMENT OF THE INTERIOR 
James G. Watt, Secretary 

BUREAU OF MINES 
Robert C. Norton. Director 



As the Nation's principal conservation agency, the Department of the Interior ^\ 
has responsibility for most of our nationally owned public lands and natural 
resources. This includes fostering the wisest use of our land and water re- 
sources, protecting our fish and wildlife, preserving the environmental and 
cultural values of our national parks and historical places, and providing for 
the enjoyment of life through outdoor recreation. The Department assesses 
our energy and mineral resources and works to assure that their development is 
in the best interests of all our people. The Department also has a major re- 
sponsibility for American Indian reservation communities and for people who 
live in Island Territories under U.S. administration. 






,4 



This publication has been cataloged as follows 




Kalyoncu, R. S 










Chemically bonded refractories— a 


review 


of the state 


of the 


art. 










(Bureau of Mines information circular 


; 8878) 








Includes index. 










Supt. of Docs, no.: I 28.27:8878. 










1. Refractory materials. I. Title. 


11. Series: 


Information cir- | 


cular (United States. Bureau of Mines) ; 


3878. 








-TN355-.U4-[TN677.5] 622s [666'.72] 


81-607575 


AACR2 





For sale by the superintendent of Documents, U.S. Government Printing Office 
Washington, D.C. 20402 



CONTENTS 



Page 



Abstract 1 

Introduction 2 

Chemical bonding 2 

Phosphate bond 3 

Phosphate bonding agents 3 

Fundamental studies 4 

Properties of phosphate-bonded materials 6 

Silicate bond 9 

Alkali silicates 9 

Ethyl silicate 10 

Oxychlor ide , oxysulf ate and oxynltrate bonds 11 

Conclusions and recommendations 13 

Bibliography 15 

ILLUSTRATIONS 

1. Compressive strength of magnesite body as a function of average degree of 

poljrmerizatlon of (NaP03)n 4 

2. Effect of heat treatment on phase constitution of aluminum phosphate binder 8 

TABLE 

1 . Oxide reactions with phosphoric acid 5 



CHEMICALLY BONDED REFRACTOR lES-A REVIEW 
OF THE STATE OF THE ART 

By Rustu S. Kalyoncu ' 



ABSTRACT 

A major goal of the Bureau of Mines is to conserve the Nation's 
mineral resources by developing improved performance materials. In sup- 
port of this mission, a survey of the state of the art of chemically 
bonded refractories has been made, covering the scientific literature, 
government reports, and patents. This review includes research and 
development results for phosphate, silicate, oxychloride, oxysulfate, 
and other bonding agents used in refractories manufacture. 

A significant finding of the review was that references on bonding 
mechanisms, bond formation kinetics, and other important process param- 
eters and conditions were few and universally vague. As a result, rec- 
ommendations are made to expand research efforts to investigate the 
kinetics and mechanisms of reactions in chemically bonded refractories. 



^Ceramic engineer, Tuscaloosa Research Center, Bureau of ^4ines, Tuscaloosa, Ala. 



INTRODUCTION 



Improved ceramic materials are in 
constant demand for processes involving 
severe chemical, corrosive, and thermal 
environments , especially at high pres- 
sures. During the past decade, demand 
for higher quality ceramic materials has 
significantly increased. This is true, 
for example, in the steel industry where 
oxygen steelmaking has increased produc- 
tion rates and operating temperatures, 
thereby compounding the demand for basic 
refractories that can withstand higher 
temperatures for use in furnace linings, 
ladles, stacks, checkers, etc. Since the 
steel industry constitutes 65 percent 
of the refractory consimiption, efforts 
to meet industry's demands for high- 
quality refractories have increased 
accordingly. 

The development of chemically bonded 
refractories represents an impor- 
tant accomplishment in the advancement 
of the technology. Chemically bonded 
brick, also referred to as unfired 
brick, is formed with the aid of selected 
additives that set up at room tempera- 
ture and provide structural integrity, 
eliminating the need for high-temperature 
sintering. 



Chemically bonded refractories offer 
significant energy savings by eliminating 
the need for high-temperature processing. 
In addition, the many methods for modi- 
fying the chemical bond offer a large 
number of opportunities for developing 
new compositions to withstand a variety 
of severe environments encountered in 
many industrial processes. However, it 
should be recognized that chemically 
bonded refractories using calcium alumi- 
nate, sodium metasilicate, MgSO^ (mag- 
nesiian sulfate), MgCl2 (magnesium chlo- 
ride) , H2SO4 (sulfuric acid) , phosphoric 
acid, and alkali phosphates as bonding 
agents have been available for many 
years. 

This report presents a review of 
literature on the present state of the 
art of chemically bonded refractories and 
identifies areas requiring research and 
development to fulfill the need for 
improved ceramic materials. This work 
supports the Bureau of Mines' mission to 
conserve the Nation's mineral resources 
and reduce imports of critical materials 
by developing improved performance mate- 
rials and using more abundant domestic 
mineral resources. 



CHEMICAL BONDING 



Reference to a chemically bonded 
refractory made as early as 1905 (25) 2 
and claimed that a valuable refractory 
lining could be made by "mixing such sub- 
stances as magnesite, chromite, etc. with 
sodium silicate and calcium chloride." 
Unfired refractory brick was mentioned by 
MacCallum (55-56) and chemically bonded 
brick by Youngman ( 103 ) . Progress in the 
chemically bonded refractories in the 
United States began in the 1930' s, with 
R. P. Heuer dominating the patent litera- 
ture. Heuer received a number of patents 
(34, 36-40) on bonding refractory mate- 
rials with sulfates, sodium silicate, 
sulfite lye, and small additions of clay 

^ 

■^Underlined numbers in parentheses refer 
to items in the bibliography at the 
end of this report. 



or bentonite. In 1941, Heuer patented a 
chemically bonded brick (38) that was 
molded in steel cases. Two U-shaped 
steel sheets were placed in the top and 
bottom of the press mold so that, after 
forming, the brick was encased in steel 
on four sides. The expansion due to oxi- 
dation of the steel casing helped to off- 
set shrinkage at high temperatures. 

Since the 1950' s, refractory 
research has made significant advances 
with the establishment of more modern 
laboratory facilities and the participa- 
tion of scientists from other disci- 
plines, such as physics, chemistry, and 
materials sciences. These scientists 
have brought new schools of thought to 
the experimentation and interpretation of 
research results. 



Phosphate-bonded high-Al203 (alu- 
mina) refractories are being used in such 
areas .as iron-transport cars, soaking pit 
slag lines, and steel ladles. Various 



monolithic refractory linings with chemi- 
cal bonding, including hydraulically cast 
materials, are being evaluated for use in 
coal conversion process vessels. 



PHOSPHATE BOND 



Recognition of bonding properties of 
phosphoric acids and various phosphates 
is not new. Numerous processes for using 
phosphate materials as bonding agents in 
refractories have been known for many 
years. Because they possess high fusion 
temperatures, phosphate bonds have always 
been of special interest in the field of 
chemically bonded refractories and have 
been studied extensively. 

Phosphate Bonding Agents 

The first significant review article 
on phosphate-bonded refractories appeared 
in 1950 (46); in it, three methods 
of developing chemical bonds were 
described: (1) reaction of siliceous 
coii5)ounds with phosphoric acids, (2) 
metal oxide-phosphoric acid reactions, 
and (3) reaction of acid phosphates with 
the refractory grains. 

The reaction of siliceous compounds 
with phosphoric acid results in a hard 
white or translucent product (depending 
on the exact silicate composition) , char- 
acterized by a lack of crystallinity. 
Various auxiliary materials are usually 
added to alter the properties of the 
chemical bond, but the basic setting 
mechanism consists of formation of a Si0 2 
(silica) gel. However, this low-melting 
frit is not a very effective bond for 
high-temperature applications. 

A number of patents have been issued 
for refractories bonded with phosphoric 
acid. One such patent describes a ZrSi0 4 
(zircon) refractory with an alkaline, 
alkaline earth, or magnesium zirco- 
nium silicate, using HCl, H2SO4, citric, 
or phosphoric acid as the bonding agent 
(48) . Phosphoric acid gave the best 
results, presumably because of its 
greater reactivity with the silicate com- 
ponents and the higher viscosity of its 
melts. Other silicates, such as those of 



Al, Cr, and Mg, react with phosphoric 
acid to form a chemical bond at about 
200° C (J_0). 

Phosphoric acid forms bonds through 
reactions with the cationic as well as 
silicate groups. For example, ZrSiO^ 
appears to form zirconium phosphate as 
well as silicon phosphates, and may form 
double phosphate salts of silicon and 
zirconium as well (64) . Aluminum, chro- 
mium, and magnesium oxides are also known 
to react with phosphoric acid at 200° C 
to form chemically bonded materials. 
These metal-phosphate reaction products 
have been found to be refractory and 
stable (thermally, chemically, etc.). 
Instead of the oxides, the halides of ^fe, 
Sn, Th, Ca, Ba, Al , Zr, or Ti may be used 
with phosphoric acid to form a chemically 
bonded refractory (65). Aluminum hydrate 
may be used with refractory clay, filler, 
and phosphoric acid to form a bond that 
becomes permanent when heated to 100° to 
300° C. 

A third method of using phosphates 
in refractory chemical bond formation is 
by the direct addition of monobasic or 
dibasic phosphates. Either alkaline 
earth acid phosphates or ammonium acid 
phosphates with aluminous materials may 
be used in place of phosphoric acids. In 
fact, since the reaction with phosphoric 
acid is very rapid, the use of phosphates 
of alkaline and alkaline earth metals is 
preferred. Even more preferable is the 
use of various organic derivatives such 
as hydrazine, hydroxylamine, aniline, 
methylamine, or ethylamine acid phos- 
phates (101). Acid phosphates may also 
be formed by mixing triphosphate with an 
acid to form monophosphate or diphos- 
phate. This process may be used with 
alkaline earth phosphates, such as cal- 
cium, which are less costly than other 
materials (50). 



It should also be noted that phos- 
phate bonding agents have been used 
for other types of applications. Sodivnn 
polyphosphate (Na4P20-7-NagP40 , 3) , an 
inorganic colloid, is used to disperse 
TiOj for casting because it improves 



green strength ( 100 ) . 
alkali silicate binders 
to be improved by 
additions. A simimary 
tions with phosphoric 
reaction products is 
table 1. 



The strength of 

is also reported 

alkali phosphate 

of oxide reac- 

acid and their 

presented in 



The use of alkali metaphosphates as 
chemical bonding agents in refractory 
mortars has been studied by Herold and 
Burst (33). Sodium hexametaphosphate 
(NagPgOjg), forms rubberlike polymers and 
yields high-strength mortars with fire- 
clay aggregates. These binders are com- 
monly used in high-Al203 refractory mor- 
tars and ramming mix. 

Effects of average degree of poly- 
merization (n) of vitreous sodium poly- 
phosphates [(NaP03)n] have also been 
investigated (73) . Maximum strength was 
attained on samples cured at 800° C, with 
an average degree of polymerization of 
24, as depicted in figure 1. Strength 
was higher when 4.3 weight-percent phos- 
phate was added as an aqueous solution 
than when 5 weight-percent was added as a 
finely divided powder. 



150 



130 




110 



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200 


1 


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180 


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V 


140 


- ^ 


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1 1 


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20 (24) 



40 



60 



AVERAGE DEGREE OF POLYMERIZATION, n 

FIGURE 1. - Compressive strength of magnesite 
bodyaSQ function of average degree of 
polymerization of (NaP03)n. A, HaPOj 
added in pov^der form; B, NaPOs added 
as water solution. (73) 



Fundamental Studies 

Published literature describing fun- 
damental studies of chemical bonding in 
refractories is almost nonexistent. How- 
ever, there have been several attempts to 
explain the kinetic processes (9, 47). 
Attempts to better understand bonding 
mechanisms, chemical kinetics of bond 
formation, and the conditions governing 
these processes have been very limited. 
A clear understanding of these fundamen- 
tal parameters has not been achieved. 



The proprietary nature of the 
refractories technology discourages pub- 
lication, for fear of losing the com- 
petitive advantage. A large volume 
of patent literature exists on the 
subject (50-51, 55, 6]_, 63-65), but 
emphasis is on the mechanics of refrac- 
tory preparation rather than the science 
of chemical bonding or the fundamental 
processes. 



TABLE 1. - Oxide reactions with phosphoric acid 



Oxide 



Time of 

setting, 

hours 



1/30 

1/30 

NS 

NS 
1/30 

NS 

12 

NS 

NS 

3 
1/2 

1/20 



3 

NR 
NS 
NS 
NR 
NS 
NR 
1/60 



NS 
NR 
24 

NR 
NR 
72 

1/60 
1/60 
1/6 

1/30 
NR 
NR 
NR 

12 
NR 
18 
48 

NR 

1/6 

NR 



Temp, 
rise of 
0.5 cm^, 

° C 



Modulus 

of 

rupture, 

psl 



Product reported in 
chemical literature 



Other data and observations 



BeO 

Be(6H)2 

MgO 

Mg(0H)2 , 

MgO calcined at 
1,280° C. 

CaO calcined at 
1,100° C. 

CaO calcined at 
1,100° C; liquid con- 
tained 9.6 pet CaO. 

SrO calcined at 
1,400° C; liquid con- 
tained 9 pet SrO. 

BaO calcined at 
1,400° C; liquid con- 
tained 9 pet BaO. 

CuO < 

CdO 

ZnO calcined at 
1,100° C. 

SnO 

SnOj 

HgO 

Hg20 

NiO 

PbO 

Pb02 

Pb304 

B2O3 

Al^Oj 

Al203'xH20 

CO2O3 

Cr203'xH20 

^^203 

^6304 

La203 

La203 calcined at 
1,400° C. 

1^203 

Si02 

H2Si03 

Ti02 

Ti(OH)^ 

Zr02 

ZrCOH)^ 

Th0 2 from Th(N0 3)4 

at 300° C. 
Ce02 

V2O5 

Cr03*xH20; M0O3; 
W0 3'XH20. 

ND Not determined. 
NR No reaction. 
NS Not set. 



15 

18 
30 
ND 
25 

ND 

24 



6 
15 



27 



3 



23 

24 



7 



38 



5 

1 



2 

36 
36 
18 

30 



2 

5 
2 



30 





750 

570 
ND 
ND 

500 

ND 
520 

ND 



570 
700 



850 



100 
ND 
ND 
ND 
ND 
ND 
ND 
ND 



ND 

ND 

1,260 

ND 
ND 
ND 

300 
ND 
400 

ND 
ND 
ND 
ND 

200 
ND 

250 
ND 

ND 

180 
ND 



Be(H2P04)2; 
Be(H2P04)2-BeHP04 

do 

MgHPO^; Mg(H2P04)2 

do 

do 



Ca(H2P04)2"xH20. 
do 



None. 



.do. 



CuHP04-H20. 
Cd(H2P04)2. 



SnHPO^ 

None 

Hg3(P04)2.. 

Hg3P04 

None 

Pb3(P04)2.. 
None 

Pb(H2P04)4 



H2BO3 

None 

Al(H2P04)j. 



None. 



.do. 



FeH3(P04)j-2-l/2H20; 
Fe(H2P04)3. 

None 

La2(HP04)j 

do 



Y2(HP04)3. 

None 

do. .. 



do 

TiOHPO^ 

None 

Zr(HP04)2 

Th(HP04)2-H20. 



None 

V0 2H2P04'4-1/2H20. 



Normal reaction. 



Do. 



Violent reaction. 

Do. 
X-ray pattern shows MgO and 
weak lines for Mg(H2P04)2. 
Violent reaction. 

X-ray diffraction shows crys- 
talline pattern. 

Violent reaction. 



Do. 



Normal reaction. 

X-ray pattern shows absence of 

crystalline Cd3(P04)2. 
X-ray pattern shows absence of 

crystalline Zn3(P04)2, 0.0, 

2, 4H2O. 
None. 

Do. 
X-ray pattern shows Hgj(P04)2. 
X-ray pattern shows Hg3P0^. 
None. 

X-ray pattern shows Pb3(P04)2. 
None. 
Cracked on setting; X-ray 

pattern shows crystalline 

product containing 

Pb3(P04)2; Pb02 absent. 
None. 

Do. 
X-ray pattern shows amorphous 

product. 
None. 

Do. 
Tacky product. 

None. 

Violent reaction. 

None. 

Shrinkage cracking. 
None. 

Do. 

Do. 

Do. 

Do. 

Do. 
Tacky product. 



None. 



Do. 
Do. 



Source: Reference 47. 



Properties of Phosphate-Bonded Materials 

With the proper selection of the 
bonding material and aggregates, 
phosphate-bonded materials do not exhibit 
reduced strength on heating. They remain 
highly refractory and possess good abra- 
sion and slag resistance after heating. 
Alumina-phosphoric acid ramming composi- 
tions are particularly resistant to Fe203 
slags at temperatures up to 1,350 C 
(62) . Cement-free phosphate-bonded cast- 
ables vary in their properties depending 
on the type and amount of bonding agent 
and the type and grading of the aggre- 
gate used. It is reported that tabular 
Al203-based castables show a reduction in 
hot strength above 800° C. This decrease 
becomes even more severe for castables 
containing MgO as a setting agent. This 
type of castable, however, is widely used 
in chemical plants because of its chemi- 
cal durability. Silicon carbide (SIC) is 
added to phosphate-bonded high-Al20 3 
products to increase their hot strength. 
This increase is thought to be due to 
formation of Si02 phosphates in the 
presence of SiC. 



Erosion resistance of AI2O3 cast- 
ables has been improved through the use 
of phosphate bonding (27). The use of 
phosphoric acid is claimed to result in 
much higher strengths than the use of 
metal phosphates such as aluminum phos- 
phate. Phosphoric acid is the preferred 
binder for attaining maximum bond 
strength, and the hygroscopic tendencies 
of these compositions can be eliminated 
by curing at 650° F. High bond strength, 
dimensional stability, and resistance to 
erosion are retained to temperatures of 
3,400° F in these compounds, and resist- 
ance to erosion is improved by about an 
order of magnitude over the existing 
commercial erosion-resistant castables. 
Stiffening and subsequent loss of work- 
ability observed in phosphate-bonded 
high-Al203 refractories (53) is believed 
to be caused by the precipitation of 
insoluble aluminous orthophosphates form- 
ing as a result of the reaction of acid 
salts with Al203-bearing materials in the 
mix. The use of inhibited phosphoric 
acid as the bonding agent (53-54) pre- 
vents this loss of workability. 



The AI2O3-H3PO4 reaction is reported to have the following sequence: 

AI2O3 + 6H3PO4 > 2A1(H2P04)3 + 3H2O; 

A1(H2P04)3 < -^ AlP04»xH20 + 2H3PO4; 

257° C 
2A1(H2P04)3 > Al2(H2P 207)3 + 3H2O; 

500° C 
Al2(H2P 207)3 > [A1(P03)3]^ + 3/2XH2O. 



(1) 
(2) 

(3) 

(4) 



The orthophosphate A1(H2P04)3 is water 
soluble and, as the bonding phase, is 
sticky and very viscous. It is a pre- 
cursor to Al2(H2P207)3 and A1(P03)3 in 
the cured refractory. 

Prevention of softening requires 
stopping or slowing down the reaction 
described in equation 2. This is accom- 
plished in one of two ways: The AI2O3 
surfaces are coated with a nonreactive 
substance that prevents H3PO4 from react- 
ing with the AI2O3, which keeps the 
pH low with excess H3PO4 and shifts 



equation 2 to the left to retain soluble 
acid phosphate; or a sequestering agent 
is used to hold the aluminum in solution 
to prevent AIPO4 precipitation. 

The volume stability is measured 
either by creep under load or by reheat- 
change at high temperatures and is an 
important performance criterion in many 
refractory applications. The volume 
stability of burned and unburned 
phosphate-bonded high-Al203 brick 
was determined by Baab and Blackwood 
(2). The authors concluded that 



phosphate-bonded high-Al203 refractories 
had poor high-temperature volume stabil- 
ity, compared with conventionally made 
brick, with corresponding AI2O2 contents. 



Figure 2 summarizes the phase con- 
versions in an aluminum phosphate binder 
with a molar ratio of P2'-'5 (phosphorus 
pentoxide) to AI2O3 of approximately 2.3 
(49). The diagram provides a general 
reference for the various phases that may 
be produced and the approximate tempera- 
ture ranges over which phase transforma- 
tions or conversions take place. As 
shown, extensive physicochemical changes 
can take place upon heating the aluminum 
phosphate phase. It is generally agreed 
that the hydrated aluminum phosphate 
phase, AlH-j(P04 )2 '3112 0, is the major 
phase producing chemical bonding. Upon 
further heat treatment, this phase is 
eventually converted to AlPO^ (berlinite 
and cristobalite forms) and A1(H2P04)3. 



Orthophosphate 
with SiO, 



AIPO4 is isostructural 
'2 ^—' ^^ ) ^'^<1 shows similar 
inversions to the alpha and beta forms of 
quartz, tridymite, and cristobalite. The 
compound A1(H2P04)3 is a highly hygro- 
scopic phase (78), which is converted to 
an amorphous phase above 570° F. Dehy- 
dration processes are completed between 
925° and 1,470° F. A glassy metaphos- 
phate phase appears above 2,000° F, 
decomposing to AlPO^ , with evaporation of 
4 is reported to be stable 
before decompos- 
^205 



P2O5. 



The AlPO 
up to at least 3,200° F 
2O3 



vapors. 

Hot gunning materials with phosphate 
binders for use in the maintenance of 
basic oxygen furnaces (BOF) are commer- 
cially available (104 ). Operator- 
controlled variables, such as moisture 
content and distance from the lance to 
the wall, contribute significantly to the 
performance of these phosphate-bonded 
gunning mixtures. Aggregates from 
reclaimed BOF brick containing carbon 
demonstrate improved adherence between 
the gunned material and wall, compared 
with conventional aggregates. Comparison 
of the amount of bonding agent with 
strength data shows that as the quantity 
of bonding agent increases, the cold 
strength increases (54). However, hot 



modulus of rupture (MOR) decreases with 
increasing quantity of bonding agent 
after an optimum 2.25 percent, for com- 
mercially available sequestered phosphate 
binders in basic compositions. The 
short-chain phosphates give the highest 
hot MOR. However, it should be noted 
that the moisture content and chain 
length also play very important roles in 
the mechanical stability of the cement. 
Short-chain glassy phosphates (n=7 
sequestered phosphate) give optimum 
results at moisture levels of 3 percent, 
and bond levels of 2.05 percent. 

Phosphate-bonded gunning mixtures 
(guncretes) are widely used for hot 
repairs of Si02 structures in coke ovens 
at temperatures above 600° C, with very 
favorable results (94). 

Zirconia (Zr02) refractories with 
phosphate bonding agents are receiving 
increasing use because of their high 
refractoriness and low thermal conductiv- 
ity (J^)» Small additions of metallic 
powders, such as nickel, further increase 
the strength and thermal shock resistance 
of these compositions. Rate of heating 
in the early stages of the curing process 
is a significant factor in the develop- 
ment of final density (porosity) and 
mechanical strength. 

Low-shrinkage ramming compositions 
of high-Al203 bodies have been prepared 
from coarse-grained chamotte, clay, co- 
rundum, and phosphoric acid (96). Mull- 
ite (3Al203*2Si02) formation by the 
reaction of corundum with free Si02 is 
thought to account for the lack of sig- 
nificant shrinkage in these systems dur- 
ing service. Curing of these coiqaosi- 
tions at temperatures above 400° C 
reduces the hydration tendency of the 
AIPO4 aluminum phosphate bond. 

Hydrated alumina (Al203*3H20) reacts 
with H3PO4 without heat to form variscite 
(A1P04*2H20) and a mixture of amorphous 
products. The AI2O3 phosphate bond pro- 
duced by direct incorporation of 
Al203*3H20 into the refractory body, fol- 
lowed by flaked lime, was found to be 
much stronger than those produced with 
AI2O3 phosphate prepared separately. 



starting mixture (Pn.; AL03 = 2.3) 



(Predominant phase) 
I 
210°-290° F 

J 

AIH3(P04)2 • H2O 

I 

300°-390° F 



AI(H,POJ 



4/3 



AIPO4 

(Berlinite) 



AIPO4 

(Cristobalite) 



>1,470'' F 

I 
AIPO4 

(Tridymite) 



1,870° F 



AI(H,P0J3 

480*'-570° F 

I 
Amorphous phase 

I 
600° -750° F 



r 



1 



AIH3P30,o AKPOa)^ 

925°-1,470° F 

i 

AI(P03)3 

1,470°-1,830° F 

AI(P03)3 



Al^iH 



'0,h 



>750° F 



AUP.O,) 



1,800°-2,190° F 



AIPO4 

(Cristobalite) 



2,000°-2,370° F 

Metaphosphate glass 
I 
2,370°-2,730° F 



AIPO4 P2O5 + 

(Cristobalite) 
I 
>3,200° F 

f "^ 1 

AI2O3 P2O5I 

FIGURE 2. - Effect of heat treatment on phase constitution of aluminum phosphate 
binder. (49) 



Phosphate-bonded basic refractories 
have been manufactured from fired MgC03 
(magnesite) with high strength and 
good spalling resistance (95). These 
compositions have been used as ramming 
mixtures for high temperature furnaces up 
to 1,500° C. 



MgO 



+ 2H3P04- 



■Mg(H2P04)2 



+ HjO. 



(5) 



Forsterite refractories with magnesium 
phosphate bonds have shown increased 
strength at temperatures between 500° and 
700° C, and no signs of diminishing 
strength to 900° C ( 105 ). Refractories 
made with about 5 percent bonding agent 



exhibited the highest compressive 
strengths. Increasing the chemical bond- 
ing agent beyond 5 percent decreased the 
strength because of a "washing out" of 
the excess bonding agent, which did not 
react with the refractory matrix. 

Refractory brick produced from dense 
briquettes without chemical bonding 
agents have lower strengths than do por- 
ous briquettes containing 5 percent bond- 
ing phase. This phenomenon is explained 
by potential displacement of the bonding 
phase to grain boundaries without pene- 
tration through the grains to form an 
effective chemical bond. 



SILICATE BOND 



Sodium silicate and ethyl silicate 
[(.€2^^)/^ SiO^] are the most common sili- 
cate binders used in refractory applica- 
tions. Sodium silicate binders have been 
studied and used most extensively in 
refractories and foundry applications. 

Alkali Silicates 

Alkali silicate binders, especially 
sodium silicates, have been used in the 
formulation of protective coatings for 
refractory linings (49) , refractory ce- 
ramic foams ( 87 , 105 ) , waterproof cement 
(45), metal casting molds (42), refrac- 
tory castables (41), and ramming mixtures 
(30). 

Refractory compositions in which 
alkali silicates have been used as chemi- 
cal binders include high AI2O3 (^tL* 22l^ ' 
AI2O3 silicates (16-17, 92), mullite 
(90) , magnesium ( 30 , 81) , and several 
nonoxide refractory materials (31). 
Water glass (sodium metasilicate) has 
been used as a refractory binder for 
blast furnace slags ( 6_) , sand-clay mix- 
tures (7), and other metallurgical slags 
(28). 

Patent literature indicates that 
alkali metal silicates have been employed 



as refractory binders, usually with 
several other additives such as strength- 
ening agents, components to provide 
hydration resistance, and plasticizers 
(89-90, 92 , 97-99). Boric oxide (B2O3) 
or B203-producing compounds such as 
Na2B407 (sodium borate) or similar inor- 
ganic salts are commonly used with alkali 
silicate bonding agents. The main func- 
tion of B2O3 is to prevent hydration and 
extend the shelf life of the binder (99). 
Alkali silicates have been used in 
refractory mixtures containing mullite 
whiskers and powder (88-89) , AI2O3 whis- 
kers and powder ( 92 ) , magnesium grains 
(90) , AI2O3 cements (86), clay concrete 
(75), and various other AI2O3 silicates 
( "69 , 75-76, 80). It is also reported 
that Na2SiFg (sodium f luosilicate) is 
used frequently with water glass in cast- 
able refractory compositions ( 61 , 67). 
The addition of metal powders such as Fe , 
Cr, and Ni increases strength at high 
temperatures (57). 

Silicate bonding agents have also 
been employed with phosphate bonding 
agents in castable formulations (68). 
Refractory castable compositions, for 
example, have been formulated containing 
sodium silicate, sodium carbonate, and 
AI2O3 phosphates (5J^, ^, 23)« The use 



10 



of silicate and phosphate bonding agents 
together has been the exception rather 
than the rule. 

The use of gypsum (CaS04'2H20) in 
refractory compositions containing lime, 
calcium silicates, and dolomitic lime 
with water glass greatly retards the 
hydration of CaO and MgO in the calcium 
silicate solutions (5^). The addition of 
3 to 5 percent gypsum in such composi- 
tions increased the strength by 
33 percent. However, gypsum contents 
above 7 percent reduced the strength 
of the calcium silicate refractories 
sharply. 

Water glass has been most success- 
fully used as a bonding agent in foundry 
applications (43). The chemical bonding 
agents used in steel foundry molds 
include furane binders (such as urea for- 
maldehyde or phenol formaldehyde solu- 
tions to which furfuryl alcohol has been 
added) with 5 to 20 percent P2O5 by 
weight of the furane binder (18). 



Ethyl Silicate 

Ethyl silicate-bonded refractories 
are prepared from a slurry of refractory 
grains with ethyl silicate, containing 
amine additives. The slurry is made as 
dry as possible and poured, tamped, or 
pressed into a vibrated mold. When the 
slurry has gelled, the article is 
stripped from the mold and the volatiles 
are removed by air drying and baking the 
pressed block to 200° C. 

The use of ethyl silicate as a 
binder in refractory components is also 
discussed in the refractories literature 
(19-20, 22-24, 29, 22.-60> ^' 102 ). The 
relatively good performance of nozzles of 
mullite and Zr02 with calcined AI2O3 com- 
positions in sliding gate systems has 
been attributed to the use of ethyl sili- 
cate bonding agents (81). The ability of 
ethyl silicate-bonded refractories to 
withstand the combined effects of severe 
thermal shock and chemical corrosion is 
closely related to the fine texture of 
the AI2O3 matrix in the refractory. 



Ethyl silicate binders are 
especially appropriate for the forma- 
tion of multilayered refractory molds 
in the lost-wax process (59). Multi- 
layered molds have been prepared using 
refractory grog or powdered fused quartz 
fillers and ethyl silicate binders. 
However, the hydrolysis and conden- 
sation of ethyl silicate can affect the 
quality of the refractory products 
fabricated. 

Other organic silicate binders have 
also been prepared by reacting sodium 
silicate with ethyl silicate. The time 
of setting for the organic silicate 
formed by this reaction at room tempera- 
ture is about 90 to 100 minutes, enabling 
the product to be formed before setting 
occurs. Compressive strengths as high as 
400 kg/cm2 have been obtained using these 
organic silicate binders (60). 

When ethyl silicate is used as a 
refractory binder, it is usually pre- 
pared by the direct reaction of sili- 
con tetrachloride and ethyl alcohol. If 
the alcohol is anhydrous, the prod- 
uct is an orthosilicate (tetraethoxy si- 
lane) , with HCl gas being produced as a 
byproduct: 



SiCl4 + 4EfOH- 



•Si(OEt). + 4HC1. (6) 



However, if industrial ethyl alcohol, 
which almost always contains some water, 
is used, the product obtained, called 
technical ethyl silicate, is a mixture 
of the orthosilicate (tetraethoxysilane) 
and polysilicates (ethoxypolysiloxanes) , 
because the water present in the alcohol 
causes some hydrolysis and polymerization 
(21). When used by itself, ethyl 
silicate has no bonding ability and, 
therefore, it is necessary to treat ethyl 
silicate with water to form a gel from 
the resulting ethyl silicate hydrolysate, 
which is the actual bonding agent. Alka- 
line hydrolysis procedures are in general 
preferred when ethyl silicate is used in 
the manufacture of refractories. How- 
everj acid hydrolysis procedures are usu- 
ally preferred in foundry processing. 
The vjater for the hydrolysis of ethyl 



11 



silicate can be provided by a Si0 2 aqua- 
sol, and in this way a hydrolysate with a 
high Si0 2 content can be prepared (2_2) . 

By using strongly basic amines with 
the ethyl silicate, intricate refractory 
shapes can be cast to close tolerances 
(85) . A few examples are electric fur- 
nace element carriers, crucibles, and 
glass feeder ware, such as plungers and 
orifice rings (86). Most refractory 



materials are suitable for use with mix- 
tures of ethyl silicate and highly basic 
amines (amine-modif led ethyl silicate) . 
Included among the frequently used cast- 
able refractory materials are AI2O3 and 
AI2O3 silicates such as sillimanite and 
mullite (87^), Zr02, ZrSi04, and SiC. 
Finished products with these compositions 
have high dimensional accuracy and excel- 
lent surface finish, as well as good 
resistance to thermal shock. 



OXYCHLORIDE, OXYSULFATE, AND OXYNITRATE BONDS 



Magnesium oxychloride cement is the 
product obtained when MgO and solution of 
MgCl2 react together, Magnesite is cal- 
cined so as to give a lightly burned 
reactive product which is ground and 
mixed as required with a strong solution 
(about 20 percent anhydrous salt) of 
MgCl2. Combination of MgO and MgCl 2 
takes place with the evolution of heat 
resulting in the formation of magnesium 
oxychloride (3MgO«MgCl2*nH20) (68). Tlie 
aged oxychloride cement appears to be 
composed of varying-sized particles of 
Mg(0H)2 (magnesium hydroxide) from which 
radiate a large number of fine needlelike 
crystals of oxychloride, which bond the 
material together. 

Addition of MgCl2 solution to MgO 
powders provides appreciable strength 
through the formation of cementitious 
phases at the grain boundaries. The dis- 
sociation of the bond phase occurs over a 
wide range of elevated temperatures, with 
loss of water at lower temperatures and 
loss of HCl at higher temperatures, leav- 
ing only MgO as the residual phase (14) . 
The system MgO-MgCl2-H20 has been the 
subject of numerous investigations since 
the discovery of the hydraulic properties 
of MgO and MgCl2 mixtures in water during 
the 1800' s. The compounds 5Mg(0H)2 
•MgCl2*nH20 and 3Mg(0H) 2*MgCl2"nH20 have 
been identified as the cement-forming 
compounds ( n_, _52^, ^, 83 ) . 

A similar magnesium oxysulfate 
cement is used as a binder in many 
structural materials and for refractory 
applications. Solutions of MgSO^ react 
with active MgO to form the cementitious 



phases, 3Mg(0H) 2'MgS04'nH20 and 5Mg(0H)2 
•MgS04*nH20, identified as the stable 
phases at 25° C, with other phases formed 
at higher temperatures. Other analogous 
mixtures , such as zinc and aluminum oxy- 
chlorides , have also been studied and are 
used in a limited number of applications. 
Aluminum oxychlorides are excellent bind- 
ers for refractory aggregates at tempera- 
tures to 1,500° C. Oxybromide analogs of 
magnesium and aluminum oxychlorides have 
been prepared, but little information is 
available regarding their properties. 

Magnesium oxychloride and magnesium 
oxysulfate cement compositions have been 
the subject of numerous patents (5,12-13, 
15 , 35 ) . In most of the compositions 
suggested for refractory lining repairs, 
large quantities of hydrophillic colloids 
are used to increase the consistency and 
allow additions of sufficiently concen- 
trated MgCl2 or MgSO^ solutions to the 
dry mix, in order to exceed the critical 
MgCl 2- or MgS04-MgO ratio necessary for 
the development of a wet mix that can be 
applied by brushing or troweling. 

Good chemical bonds have also been 
obtained using nitrates [NaN0 3 or 
Ca(N03)2] in quantities of 8 to 20 per- 
cent by weight of solids, with a variety 
of constituent combinations of MgO, 
ilmenite, chromite ore, and Fe , Si, and 
Al in lesser amounts (4_) . In these com- 
positions, nitrates react quickly with 
Fe-Si , forming a silicate bond. Calcium 
nitrate Ca(N03)2 is preferred to NaNOj 
since a more refractory silicate is 
formed. 



12 



Increasing the high-temperature 
mechanical strength of cast AljOj refrac- 
tories by introducing organic additives 
such as polyvinyl alcohol, sucrose, and 
flour has not been very successful. The 
development of an organic film on the 
AI2O3 particles is thought to mask the 
intermolecular attraction forces and 
lower the strength of the cast refractory 
(44) . Additions of up to 10 percent 
AI2O3 treated with HCl solutions have 
significantly improved the strength of 
AI2O3 castings at temperatures above 
1,000° C. It is reported that the forma- 
tion of aluminum oxychloride bond on the 
surfaces of y~^12^3 particles treated 
with HCl solution produces higher 
strength. 



The dissolution of >feO from the com- 
plex is essential in the hardening of 
both chloride and sulfate cements of mag- 
nesia. Setting processes involve forma- 
tion of Mg(0H)2 for sulfate cement and 
formation of basic MgCl2 for chloride 
cement. The agglutination of the fine 
particles in the cement mixture is 
explained by hydrogen bonds (32) acting 
directly between the OH groups of the 
I-lg(OH) 2 in one case and of the basic 
MgCl2 in the other. 

The setting of chloride cements can 
best be illustrated by the following 
chemical reaction where 3Mg(0H) 2*MgCl2 
•nH20 forms as the bonding agent: 



CI 2 
Mg(OH)x 
(H20)4-x 



Hx + 3Mg(0H) 



The use of the so-called "salt 
phase" as an inherent body component is a 
new element in the development of manu- 
facturing procedures for lime-base 
refractories. The salt phase is mainly 
CaClj (calcium chloride), which melts at 
7 72° C but can be lowered by as much as 
400° C in the presence of other salts. 
The salt phase melts at low temperature, 
yielding a reactive liquid of low viscos- 
ity, and leaves the system gradually as a 
result of high-temperature hydrolysis. 
The formation of 4CaCl2'CaO upon heating 
and its effects on the subsequent ceramic 
processes is thought to be responsible 
for the development of a unique micro- 
structure and the high-temperature volume 
stability (64). The volume stabilization 
is believed to be helped by the pro- 
gressive evolution of the HCl resulting 
from the high-temperature hydrolysis of 
chloride salts. An even more pronounced 
effect on volume stabilization has been 
observed in bodies with CaCOj additions 
(along with CaCl2), the so-called calcite 
brick. As more gas phase (CO 2) is cre- 
ated by the decomposition of the car- 
bonates, and if the viscosity of the melt 
is increased (by addition of silicates), 
a marked expansion of the products may 
occur. 



CI 2 

Mg(OH) 

H2O 



•(MgOH)3'3H20. 



(7) 



The strength of unfired refractories 
containing magnesium oxysulfate, mag- 
nesium oxysulf ate-H3B03 (boric acid), and 
sodium polyphosphate bonds has been 
determined as a function of temperature 
(74). All the bonding agents develop 
higher strength in the presence of chro- 
mite, and the addition of H3BO3 with 
MgS04*7H20 increased the strength of the 
refractory in the 400 to 900° C range. 
Above 1,000° C, the strength of these 
same refractories was significantly 
decreased due to incongruent melting of 
magnesium metaborate (79). 

One of the problems encountered in 
the use of MgCOj refractories is the par- 
tial hydration of MgO in the presence of 
water. The thermal decomposition of 
Mg(0H)2 upon heating to 400 to 500° C and 
the consequent evolution of water vapor 
cause severe thermal spalling. Additions 
of approximately 1 percent B2O3, yielding 
material such as H3BO3, reduce the hydra- 
tion tendencies of the MgO refractories. 
In the presence of MgSO^ or MgSO^- 
yielding material, the addition of H3BO3 
is not only ineffective in preventing MgO 
hydration but actually increases the 
degree of hydration significantly under 
certain conditions. An improved chemical 



13 



bond that at the same time prevents MgO 
hydration has been described by Martinent 
(58)« The bonding agent consists of 
35 mesh dead-burned MgO, from 0.5 to 5.0 
percent magnesixmi sulfate heptahydrate by 
weight of MgO, and a boron compound 
yielding B2O3 upon firing, to provide a 
weight ratio of MgS04:B203 of 2:1 or 
less. This bonding composition is used 
in amounts of from 10 to 60 weight- 
percent of the total refractory 
composition. 

A patent by Montague (63) describes 
a method for obtaining superior chemical 



bonding in refractory compositions con- 
taining olivine [(Mg, Fe)2 8104]. The 
olivine fines are slurried with water, 
and then H2SO4 is added and mixing is 
continued. The reaction generated pro- 
duces large quantities of steam rapidly, 
and the mixture becomes very viscous and 
hardens into a solid cake. Ordinarily, 
the cake is crushed and screened, for 
convenience. Refractory linings of oli- 
vine, MgO, and chrome with the described 
bonding agent were found to be superior 
to similar compositions using sodium sil- 
icate bonding. 



CONCLUSIONS AND RECOMMENDATIONS 



Chemically bonded brick offers 
promise in a number of refractory appli- 
cations for iron and steelmaking, glass 
manufacturing, high-temperature chemical 
processes, and energy conversion pro- 
cesses, as well as in nonref ractory 
applications. Unfortunately, the efforts 
to explain chemical kinetics and mechan- 
ism of bond formations have been limited. 
With the exception of information on den- 
tal cements, few data regarding the bond- 
ing reactions and bond mechanisms are 
available; in addition, the identified 
references about bonding mechanisms are 
very limited. Chemical kinetics and 
important reaction parameters have not 
been systematically studied. 

The possibility of forming a large 
variety of chemical bonds is great, 
thereby extending the potential applica- 
tions for chemically bonded brick in 
severe environments at moderately high 
temperatures. Coal gasification and 
liquifaction present one area of poten- 
tial applications where the thermal con- 
ditions are moderately severe (1,100° C), 
and high chemical durability is required 
for refractory liners in reducing or oxi- 
dizing atmospheres with corrosive gases 
and liquids. 

The feasibility of using raw mate- 
rials of marginal purity, such as spent 
refractory linings and byproduct slags, 
could be enhanced through the development 
of chemical bonding agents with various 



compositions for use in high-temperature 
environments. 

Based on the conclusions outlined 
above, a number of research development 
projects are recommended: 

1. Fundamental research efforts 
should be devoted to better understanding 
chemical bond development for various 
refractory systems. Kinetics and the 
mechanism of chemical bond formation 
should be examined. A fundamental under- 
standing of the processes leading to 
chemical bond formation will identify 
opportunities for development of materi- 
als with new and improved performance, 
which in turn would help conserve the 
Nation's mineral resources. 

2. Research efforts should be 
devoted to developing more versatile and 
inert chemical bonds in chemical binder 
systems and combinations of binders. 
Attempts should also be made to determine 
mechanistically the role of each compo- 
nent in a binder system. In addition, 
the effects of important manufactur- 
ing parameters, such as curing rates, 
moisture content, and mixing methods 
for different binder compositions, 
should be determined. The role of metal 
powder additions should also be investi- 
gated, and the use of chemical bonding 
agents in combination should be 
explored. 



14 



3. Research activities for the 
development of monolithic refractories 
should continue and be expanded to 
include chemically bonded compositions in 
addition to hydraulic bonds. 

4. Research should be conducted on 
the development of chemically bonded 
refractories from spent refractories, 
waste linings, and raw materials of mar- 
ginal purity. 

5. Research efforts should be 
directed at improving the short and 
unpredictable shelf life of many chemical 
binders, which would prove very helpful 
in the development of next-generation 
chemically bonded refractory products. 

6. The opportunities for applica- 
tion of chemically bonded refractories 
can be greatly extended by solving 



certain pressing problems, such as bond 
migration, bloating, and low hot 
strengths, which have greatly limited 
their use. 

7. Pitch and tar bonding agents 
present some health and environmental 
problems; chemical bonding agents should 
be developed to substitute for these 
organic bonding agents. 

8. In many cases, the literature 
evaluation of refractory compositions has 
not included the service conditions to 
which the refractory would be subjected. 
It is recommended that any evaluation 
program following a development effort 
should consider the service conditions, 
and appropriate evaluation procedures 
should be instituted as part of all 
refractory development studies. 



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15 



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